The invention relates to a drilling vessel. The invention furthermore relates to a method for constructing a riser string. More specifically, the invention relates to a drilling vessel with a drilling deck and a tubular support structure such as a riser spider.
When assembling and de-assembling a riser string on an offshore drilling vessel, the riser string is typically supported from a riser spider in the drilling opening. The riser spider is often placed on a gimbal, located between the riser spider and the drilling deck. The gimbal is used to compensate for the movement of the vessel and evenly distributes loads on the riser spider and the riser supported by it.
The riser spider has a support opening for receiving the riser. Sliding dogs are used to adapt the support opening of the riser spider to fit the diameter of the tubular sections of the riser. Alternatively, bushings are used for adapting the size of the support opening.
Risers are composed out of multiple tubular sections that are coupled to each other via sockets, coupling sleeves or flanges. These couplings typically have a diameter larger than the diameter of the tubular sections of the riser, and are used to hang of the riser on the bushing or dogs.
When lowering a riser through the drilling opening the bushings, and sometimes even the riser spider, are temporarily removed to allow the passing of the wider parts such as the couplings and inserts provided in the riser string. Removing and re-installing of the bushings and the riser spider is time consuming and difficult in the restricted space of the drilling deck above the drilling opening. This is in particular the case when assembling and de-assembling a riser string from riser segments, which involves alternatingly supporting the riser string and passing a riser segment or insert through the drilling opening.